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The Evanescence Character Of Optical Microfiber And Research On Micropollutant Detection

Posted on:2014-06-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z T WeiFull Text:PDF
GTID:1108330479479548Subject:Optical Engineering
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Optical microfiber(OM) is the optical waveguide fiber with micron and sub-micron diameter. For characters of strong optical confinement and large evanescent field, it has been widely used in the field of optical fiber sensing, near-field optics, nonlinear optics, quantum optics. Due to the special need of high power laser system for micropollutant detection technique, this thesis firstly proposed the shceme of micropollutant sensing based on the large evanescent field character of OM, and the proposal is experimentally verified. What’s more, the OM sensor, with the characters of highly sensitive, inline, real time, and easy to multiplexing, maybe become an effective way to solve the engineering problem.The main results are summarized as follows:(1) An OM fabrication system with high performance is designed and constructed. The relationship between the shape of OM and drawing parameter is theorically deduced, which provides guidance for the OM fabrication. Then, the influence of hotzone length, gravity force, and slide guide installtion is analyzed, and the installtion technical requirements are definitely pointed out. To optimize the fabrication technics, we counteract the influences of the increasment of heating length and drawing length by choosing proper drawing parameter.(2) The principles of micropollutant detection by using OM, expecially for microparticle contaminants, airborne molecular contaminants and totally immersed contaminants, are all analyzed. The relationship between the OM additional loss and microparticles with different refractive indices, is calculated. The influences of operating wavelength and polarization state are also considered. Then, the relationship between the OM additional loss and the thickness of organic contamination film is analyzed. The influences of the operating wavelength and the OM profile shape are also taken into account. The sensitivities of micropollutant detection by cavity ring down based on OM(CRDOM), and microfiber Bragg grating(MFBG) are both theoretically calculated.(3) Experimental results have proved that the proposal of micropollutant detection by using OM is feasible. Before carrying out the experiments, kinds of nonideal facor, such as fluctuation of optical fiber coupler ratio, shape error of OM profile, temperature increasment of orgaic contamination film, and polarization state of incident optical wave, are considered and sovled. By using microparticals of dust and alumina, the proving experiments are demonstrated. A CRDOM system is constructed, and the sensitivity of particulate detection is considerly improved. The relative weighting method is used in the orgaic contamination sensing experiment, and the results got by weighting and OM are in according with each other. In addition, a mirocpollutant detection test macro is constructed. By using reference arm, phase lock, SLD source, and incident power controllment techniques, floor noise of the system is lower than 0.0002 d B, and the dynamic range of the system is higher than 30 d B, the minimum detectable thickness variation of orgaic contamination film is 0.08 nm, while the maximum detectabe thickness variation is 550 nm, the silica micropartical as small as 300 nm can be detected, and as many as 150000 of this kind microparticle can be measured. What’s more, the advantages of repetitive-use and response uniformity are proved by the orgaic contamination experiment.(4) A new method to fabricated MFBG is proposed and demonstrated, which is named three-step-method, composing etching, drawing, and wirting, and this method can write MFBG from conventional photosensitive fiber by using Kr F excimer laser. The photosensitivity of the fabricated OM is effectively reserved. An MFBG has been successfully fabricated by using a Kr F excimer laser through a phase mask with a pitch of 1089.3 nm. The concentration of the brine was measured by immersing the MFBG into the liquid. The minimum detectable refractive index variation can reach 7.2×10-5 at a refractive index value of 1.33.(5) A new method to detect the absorption coefficient of liquid is proposed and proved, and the temperature character of the absorption coeffient is also researched. The relationship between the OM additional loss and absorption coeffient is theorically analyzed. The contribution of waist region and taper region are both considered. By using pure water or fiber coating as the clad of OM, we get the relationship between additional loss of OM and temperature, and the results show well repeatability. When the OM is immsed into the pure water, the temperature sensitivity of the OM with a diameter of 1.7 μm is-0.083 d B/℃, and that for the 2.4 μm diameter OM is-0.038 d B/℃. The floor noise of the sensing system is 0.005 d B, so the minimum detectable temperature variation is 0.06 ℃ and 0.13 ℃, respectively, while the outer clad changes to fiber coating, that for the OM with a diameter of 2.1 μm can reach as small as 0.012 ℃.
Keywords/Search Tags:optical microfiber, evanescent field, optical fiber sensing, microparticle contaminant, organic contaminant, refractive index, absorption coefficient of liquid
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